The invention is concerned with an enantioselective zwitterionic ion-exchange material comprising a chiral selector component (SO) comprising at least one cation exchange group and at least one anion exchange group and a carrier, carrying said selector component directly or via a spacer.
In the field of enantioselective chromatography using ion exchanger type chiral stationary phases (CSPs) separation takes place due to enantioselective ion pairing of oppositely charged chiral selectors (SO) and analytes. Conceptually, a chiral molecular recognition process of enantioselective ion pairing could principally occur with every CSP that contains ionizable groups within its chiral selector moieties, may they be part of e.g. a small SO molecule- or of glycopeptide- or protein-based separation materials [1-4]. CSPs that are based on immobilized cinchona alkaloid derivatives as low molecular mass SOs specifically employ the ion pairing process as the primary interaction between SOs and analytes. Thereupon, secondary interactions like for instance H-bond formation and π-π stacking (interaction) can follow and finally lead to enantiodiscrimination. Among this thoroughly investigated class of chiral weak anion exchangers (WAX) the CSPs based on O9-tert-butylcarbamate derivatives of quinine and quinidine as SOs provide excellent enantioseparation capabilities for a broad range of chiral acids [5,6]. FIG. 1a shows the SO structure of a quinine-type WAX CSP which is also commercially available (CHIRALPAK QN-AX from Chiral Technologies, France).
Complementary to these WAX type CSPs, also separation materials based on fully synthetic low molecular mass weak and strong cation exchanger (WCX and SCX) SOs have been developed and investigated for the enantioseparation of chiral basic analytes via an ion pairing driven process [7-12]. Exemplarly depicted in FIG. 1b with a chiral sulfonic acid based SO, such cation exchanger CSPs have been applied to different enantioselective separation techniques including capillary electrochromatography (CEC), capillary liquid chromatography (CLC), and high performance liquid chromatography (HPLC) [13]. Only very recently, a more complex but very interesting boronic acid based chiral cation exchanger SO derived from the macrodiolide boromycin was also reported [14] indicating the potential of ion exchangers in enantioselective chromatography.
However, chiral cation and anion exchangers encounter the limitation of addressing only analytes that carry a charge of opposite sign.
Principally, stationary phases containing complementary charged groups have also been described in the literature [15-19] and were applied mainly for the separation of inorganic cations and anions. Systems employing chiral zwitterionic moieties have also been reported previously for different purposes [20-24] but were not envisioned for enantiomer separations.